The invention relates to a method and apparatus for sensing the resistance of a Programmable Conductor Random Access Memory (PCRAM) element.
PCRAM devices store binary data as two different resistance values, one higher than the other. The resistance value represents a particular binary value of logic xe2x80x9c0xe2x80x9d or logic xe2x80x9c1xe2x80x9d. When sensing the resistance value of a PCRAM device, it is common to compare the resistance of a memory cell undergoing a read operation with resistance of a reference cell to determine the resistance value of the cell being read and thus its logic state. Such an approach is disclosed in U.S. Pat. No. 5,883,827. However, this approach has some limitations.
If the reference cell is defective and a column of memory cells within an array uses a same defective reference cell, the entire column of memory cells will have erroneous resistance readings. In addition, specialized circuitry is required to write a resistance value into the reference cell, and a sense amplifier circuit for such an arrangement tends to be complex and large.
Typically, sensing schemes for PCRAM devices also tend to have a unique architecture which is different from that normally employed in typical DRAM circuits. Although PCRAM""s differ from DRAM""s in that they store binary values in resistive memory elements rather than as charges on capacitors, and although PCRAM""s are non-volatile, where the capacitor structures employed in DRAM""s are volatile, nevertheless it would be desirable if the read and write circuits for both devices were as similar as possible so that existing DRAM memory device architectures could be easily adapted to read and write PCRAM devices.
The present invention provides a PCRAM memory device and its method of operation which utilizes a read architecture similar to that employed in some DRAM memory devices. A pair of complementary PCRAM memory cells comprising first and second programmable conductor memory elements are employed, each connected to respective access transistors. During a write operation, the first and second memory elements are written with complementary binary values, that is: if the first memory element is written to a high resistance state, then the second memory element is written to a low resistance state; whereas if the first memory element is written to a low resistance state, the second memory element is written to a higher resistance state.
During a read operation of, for example, the first memory element, a sense amplifier is connected so that its respective inputs are coupled to receive respective precharge voltages which discharge through the first and second memory elements. A sense amplifier reads the discharging voltages through the two memory elements to determine which is the larger voltage, thus determining the resistance (high or low) and logic state (high or low) of the memory cell being read.